I'm thinking that the additional listenership is because these people -- who most likely work in office buildings and can't pick up AM stations inside them. I'm one of them, that's why the bulk of my daytime radio listening is FM . The bulk of my evening/nighttime listening is on AM, however -- and I'm 31 years old. If there was an FM simulcast of the AM stations I listen to, I'd tune over to that.
Maybe stations SHOULD simulcast -- that's how you get the benefits of both: Long range on AM for distant listeners and for local areas where AM is non-existent, there's the FM.
I think that's the theory behind WNYC-AM/FM's simulcast. This station dropped the daytime music shows and flipped their FM to a simulcast of the talk station on AM, which has increased their listenership, according to them.
But they've KEPT the AM instead of dumping some other format on that station (or selling it off), because they realize that it is still valuable and has its uses.
AM has little appeal due to its lousy sound
And IBOC isn't helping matters any.
On the contrary, since so few (almost nobody outside of us geeks) have HD radios, much less HD radios that work properly on AM, they're stuck with an analog signal that is now plauged by hiss from IBOC and tinny or tubby sound from mis-adjusted processors that literally causes fatigue after a short period.
The quality of modern, inexpensive AM radios leaves a LOT to be desired. Most of these things are wide as barn doors. In New York, using one of these, you're lucky if you can pick up two or three of the strong stations. But even then, you'll have them either cross-talking or showing up at multiple spots on the dial.